More Rain And Thunderstorms For Sydney Today

After a wet weekend, we're in for even more rain today, and possibly some thunderstorms before the day is out.

A low pressure system is expected to drench with 20 and 30 millimetres of rain expected, while 15 to 25 millimetres is forecast for Penrith.

Mohammed Nabi from the Bureau of Meteorology says it will be a wet morning with rain increasing throughout the day.

"It becomes a bit more prevalent during the day, people get into work and it will just seem like it's raining even more as opposed to easing off," he said.

Thunderstorms are likely across the Blue Mountains and then expected to move across the city in the afternoon.

"There's a chance of thunderstorms with the low, it's going to create a lot of instability over the region," Weatherzone meteorologist, Craig McIntosh said. Thunderstorms are likely to hit Sydney with the possibility they could be severe.

"People will probably hear a bit of thunder in the afternoon and see some flashes of lightning," Mr Nabi said.